The present invention relates to mixtures of metal powders and specifically with a method for developing and sustaining uniform distribution of first and second dissimilar metal powders, in particular metal powders having substantially differing densities.
In the prior art it is well recognized that in a mixture of metal powders having different masses (densities), it is difficult to obtain such mixing of the powders as will result in uniform distribution of the particles of one powder or powders with the particles of one or more of the other powder or powders without subsequent separation of the different metal powders according to their respective densities. This problem may occur when mixing two metal powders or when mixing more than two metal powders. Blending of the metal powders, in their dry state, is commonly used to obtain a mixture of two or more metal powders. Even with well-blended powder mixtures, substantially any vibration, even slight vibration, of the mixture can result in separation of the powders as a function of their respective densities. Consequently, division of a blended mixture of metal powders into aliquots for any of various reasons, such as for loading of a measured quantity of the powder mixture into a die cavity, frequently results in material differences in the overall density of the aliquots. Moreover, the powder distribution within the aliquot ceases to be uniform, resulting in a non-uniform density distribution within the resulting die-pressed product.
In the art of manufacturing powder mixture-based ammunition projectiles, it is common to press a measured quantity of the powder mixture in a die cavity to obtain a self-supporting core from which the projectile is subsequently produced. Accuracy of delivery of the projectile to a target and repeatability of performance of the projectile with respect to its muzzle velocity, its flight characteristics to a target, and/or its terminal ballistics are highly desired and in certain instances, extremely critical. Use of this type ammunition in law enforcement and military applications demands very exacting standards of performance of the projectile and the same performance from projectile to projectile. The present inventor has found that even small disparities in the desired mass (density) of each projectile and like small disparities in the uniformity of density distribution of the formed projectiles can be unacceptable.
Prior to the present invention, it has been proposed that projectiles for gun ammunition be xe2x80x9cpowder-basedxe2x80x9d, that is, the projectile is formed by die-pressing one or more metal powders into a self-supporting compact. This activity is fraught with problems and/or difficulties, such as choice of powders, uniformity of mixing of the metal powders, sustaining uniformity of mixing of the powders during die-loading procedures, selection of pressures and techniques, such as selection of die lubricants and/or sintering or the like, to ensure production of a self-supporting compact suitable for further processing, and many other problems and/or difficulties. As noted hereinabove, a major concern in the manufacture of powder-based projectiles for gun ammunition relates to the uniformity of density distribution of the powders within a mixture within the projectile. The present inventor has discovered that in the production of projectiles for use in gun ammunition, the performance of the projectiles, when incorporated into a round of ammunition and fired, is a function of the uniformity of distribution of the density of the projectile, both the density distribution in a direction radially of the longitudinal centerline of a cylindrical compact which is to be incorporated into a projectile and the location of the center of gravity of the projectile (nutation effect). Very importantly, every projectile of a desired given size, weight, density distribution, etc., needs to be consistently the same from projectile to projectile.
In the prior art, it has been suggested that a mixture of tungsten metal powder be mixed with a lighter metal powder, such as tin, lead or the like, be used in the production of a powder-based gun ammunition projectile. The difference between the densities of these two metal powders (or other mixtures of two or more metal powders) gives rise to serious and deleterious separation of the tungsten particulates from the tin particulates within a mixture of these two metal powders into striations or layers of primarily tungsten particulates and tin particulates. This separation of the powders (1) precludes division of the powder mixture into aliquots of a given quantity of tungsten powder and a given quantity of tin powder, and (2) uniformity of density distribution of the powders as the mixture is vibrated, etc. in the course of the aliquoting and in the process of pouring the powder into a die cavity.
It is therefore an object of the present invention to provide a method of developing and sustaining uniform distribution of the individual particulates of a plurality of metal powders in a mixture thereof.
It is another object to provide a method for obtaining aliquots of a powder mixture wherein each of the aliquots is of essentially equal density and exhibits essentially uniform density distribution throughout the aliquot.
It is another object to provide an improved die-pressed compact for use in the manufacture of an ammunition projectile.
It is another object to provide a method for the production of a plurality of ammunition projectiles having essentially equal density and each having essentially uniform density distribution throughout the projectile.